Biological cells are generally highly selective as to the molecules that are allowed to pass through the cell membrane. As such, the delivery of compounds, such as small molecules and biological molecules into a cell is usually limited by the physical properties of the compound. The small molecules and biological molecules may, for example, be pharmaceutically active compounds.
The lack of delivery of such molecules, including macromolecules, such as proteins and nucleic acids, into cells in vivo, has been an obstacle to the therapeutic, prophylactic and/or diagnostic use of a large number of potentially effective compounds. In addition, many compounds which appear promising in vitro, have been discarded as potential drugs due to the lack of ability to deliver the compound effectively inside a cell, in vivo.
Several reports have addressed the problem of delivering compounds to cells by covalently attaching the compounds to “protein transduction domains” (PTDs). Schwarze et al. (Trends Pharmacol Sci. 2000; 21:45-8) and U.S. Pat. No. 6,221,355 to Dowdy disclose several PTDs that can cross the lipid bilayer of cells in a concentration-dependent manner. The PTDs disclosed include PTDs derived from the HIV-1 tat protein, from a Drosophila homeotic transcription factor encoded by the antennapedia (abbreviated ANTP) gene, and from a herpes simplex virus VP22 transcription factor. The HIV-1 tat PTD is eleven amino acids in length, the ANTP PTD is sixteen amino acids in length, and the VP22 PTD is 34 amino acids in length.
Recent publications, however, indicate that these PTDs enter cells via energy-dependent endocytosis. Therefore, the “PTD-cargo” complexes are contained within the cell's endosomal vesicles and not available to, for example, the cytoplasm of the cell. Accordingly, the “PTD-cargo” complexes must be released from the endosomal vesicles in order to be bioactive (Richard et al., J. Biol. Chem. 2003; 278:585-590; Drin et al., J. Biol. Chem. 2003; 278:31192-31201). Further, there are recent reports that these PTDs are toxic to cells.
Thus, there is a need for peptides which are capable of crossing the lipid membrane of cells in an energy-independent non-endocytotic manner. In addition, in order to avoid immune responses, commonly known for large peptides, there is a need for smaller, peptidase-resistant, peptides. Finally, it is important that the peptide carriers be nontoxic to cells.